The present invention relates to electronic analog to digital converters producing a straight binary output equal to, in binary code language, the value of the analog input.
Basically, the analog input is an electric signal having a magnitude which may be steady or may vary. The output thus is a digital word in the form of the number of bit outputs representing the magnitude of the input at any one time, in binary form, i.e., 2.sup.0, 2.sup.1, 2.sup.2, 2.sup.3, . . . , 2.sup.n. Each output is either a logic 1 or a logic 0. For an output voltage having logic 0 or 1, the digital word is equal to: EQU E.sub.0 (2.sup.0) + E.sub.1 (2.sup.1) + E.sub.2 (2.sup.2) + E.sub.3 (2.sup.3) + . . . + E.sub.n.sub.-1 (2.sup.n.sup.-.sup.1)
The above expression is called the binary code and the number of outputs n is called the number of bits.
In a one bit analog to digital converter the output would have a binary coded 0 or 1 corresponding to zero or 2.sup.0 respectively. A two bit system would have two outputs, each of which could be a 0 or 1. These correspond to binary code of 2.sup.0 and 2.sup.1. Thus the numerical value of the output of a two bit system would be 0,2.sup.0,2.sup.1, and 2.sup.0 + 2.sup.1, corresponding to magnitudes of 0,1,2, and 3 respectively. In an analog to digital converter, the maximum analog input signal acceptable has a certain value which represents the full scale (hereinafter abbreviated FS) analog input voltage. Thus in a two bit analog to digital converter, there being four output numerical values, the digital output would represent input values as follows: 0,FS/4,FS/2, and 3FS/4 where 0 represents 0, 2.sup.0 represents FS/4, 2.sup.1 represents FS/2, and 2.sup.0 + 2.sup.1 represents FS/4 + FS/2 or 3FS/4. The 2.sup.0 bit is called the least significant bit or LSB and the 2.sup.1 bit (2.sup.n.sup.-1 bit) is called the most significant bit or MSB. It should also be pointed out that the maximum digital word output corresponds to an input analog value equal to or greater than Fs - 1 LSB. Thus the binary digital output word 11 corresponds to an analog input value equal to or greater than 3FS/4.
Similarly in a 10 bit analog to digital converter there is 2.sup.10 or 1,024 discrete input analog voltage level which may be represented by the binary digital output. The MSB, which would be the 2.sup.10 bit, represents 0.5FS and the LSB represents the 2.sup.0 bit or (1/1024)FS, a value of less than 0.1%FS. The number of discrete levels or values which may be digitally expressed is equal to 2.sup.n, where ( n) is the number of bits. This is called the resolution. In the ten bit analog to digital converter example above, the resolution would be the 0.1% of FS.